Anata wo Shinjiteru - Samishii Kodomotachi

From momiji_mii's livejournal: A story about a teenage girl whose life has become a living hell because of her alcoholic father. Her mother seems to love her little brother better than her and she feels a lot of pressure because of the circumstances of her family. She's definitely more grown-up than the rest of the girls in her class. She lately feels some sort of connection with one of the bad guys in school, who has been showing kindness to her younger brother lately. He takes both of them to the zoo after the father breaks his promise due to being hungover.